1. Field of the Invention
The present invention relates to clinical diagnostic processes, and more particularly to a system and method of determining a sigma-metric for such processes.
2. Description of Related Art
The sigma-metric was first introduced by Motorola as part of its Six Sigma Quality Management program. While initially applied to manufacturing processes to reduce defects and improve quality, the six sigma principles are today widely used throughout various aspects of manufacturing and business to improve processes. The sigma-metric defines how many sigmas (i.e., standard deviations) of deviation or variation a process can experience and still be within its allowable tolerance limits. The higher the sigma, the more robust a process is in the presence of error. By definition, a six sigma process is still within specification even with six standard deviations of variation.
Use of the sigma-metric allows quality comparison of widely divergent processes. For example, as cited in the article Six Sigma Quality Design & Control, (WI:Westgard QC, Inc., 2001, pg 29), the sigma-metric of various processes are: Airline Baggage Handling—4.2 sigma; Airline Passenger Survival—6.42 sigma; Hematology Specimen Acceptability—4.15 sigma; and Firestone Tires—5 sigma. The author of that article, Dr. Westgard, introduced the six sigma concepts to the clinical diagnostic community with his essay Six Sigma Quality Management and Desirable Laboratory Precision (2003, Westgard QC, www.westgard.com/essay35.htm).
Current approaches to computing the sigma of clinical processes assume homoscedasticity—or uniform variance, even though it is widely known that clinical processes are rarely homoscedastic. In fact, it is common in clinical diagnostic processes to have different variation at different concentrations. Because of that variation, using conventional methods of determining sigma-metrics results in multiple sigmas applying to a single clinical process. It is thus unclear which of those sigma (if any) is correct for that clinical process, and how to use the sigma-metric when various sigmas are determined for a single clinical process. Current practice is simply to display the separate sigma for each level of control material.